This invention relates to a method of controlling a searcher used in a CDMA (Code Division Multiple Access) system and in particular, to a receiver section of a base station and/or a mobile terminal used in the CDMA system A recent attention tends to be focused on a CDMA system as a radio communication system which carries out communication between a base station and a plurality of mobile terminals by a radio signal within a service area or cell. This is because communication between the base station and a plurality of mobile terminals can be carried out by the same frequency within the same cell and, as a result, frequency resources can be effectively used in the CDMA system.
As such a CDMA system, a direct sequence (DS)-CDMA system has be known which directly modulates an information signal into a spread signal by a specific spread spectrum code unique to each mobile terminal. The DS-CDMA system makes it possible to spread the information signal into a very wide band. More specifically, communication from the base station to each mobile terminal is carried out through a forward link while communication from each mobile terminal to the base station is carried out through a backward or reverse link, in general, the forward link includes a pilot channel and a traffic channel while the reverse link includes a traffic channel without any pilot channel. At any rate, a receiver section of the base station and the mobile terminal must identify each channel from one another to detect a counterpart side on the communication.
In addition, it is to be noted in the CDMA system that the receiver section of each of the base station and the mobile terminal is given reception signals through different propagation paths which may be collectively called a multi-path and, therefore, the reception signals are inevitably subjected to interference on passing through the different propagation paths. Under the circumstances, the receiver section should reliably detect its own channel from the reception signals received through the multi-path. Taking this into consideration, the receiver section in the CDMA system is equipped with a searcher for searching its own channel which is arranged within a time slot predetermined for the receiver section.
In the interim, the abovementioned DS-CDMA system includes a mobile communication system of DS-CDMA which is implemented in compliance with the IS 95 standard prescribed by ANSI (American National Standards Institute). In the mobile communication system, communication from each mobile terminal to the base station is carried out through the reverse or uplink by using a sequence of burst like frames from each mobile terminal. This mobile communication system is helpful to save power consumption in each mobile terminal. Specifically, each of the frames is divided into sixteen slots for arranging an audio data signal or any other data signal given in a digital signal. In this event, the digital signal is usually obtained by encoding a sound signal into digital samples by a codec and by carrying out predetermined code conversion about the digital samples by a vocoder induded in the mobile terminal. Herein, it is to be noted that the digital samples are converted by the vocoder into different rates of the digital signal, as an amount of the digital samples is varied in each frame.
The digital signal of different rates is arranged in each frame at different frame rates determined by the number of data transmission slots in each frame. As a rule, the frame rates are classified into four rates, namely, a full rate, a half rate, a quarter rate, and an eighth rate. The digital signals are arranged in all of the slots in each frame at the full rate while the digital signals are arranged in half slots of sixteen slots at the half rate. Likewise, the quarter and the eighth rates are for arranging the digital signals in four and two slots included in each frame, respectively.
In U.S. Pat. No. 5,535,239 issued to Padovani et al, disclosure is made about a data burst randomizer which is used in a transmission system and which serves to determine power control groups in accordance with a predetermined algorithm by monitoring predefined bits in a previous frame However, Padovani et al never point out any problem which might occur on reception of the digital signals of variable frame rates.
Practically, when such digital signals of variable frame rates are received by a conventional receiver section, a searcher in the conventional receiver section is put into an active state, regardless of the frame rates of a reception signal. In other words, the searcher is always operated in tune with the full rate. Accordingly, when transmission is made at any other rates than the full rate, the searcher in the receiver section is uselessly operated and gives rise to a useless operation of any other peripheral circuits related to the searcher for a quiescent period namely, an unvoiced period. In other words, the probability of searching an unvoiced slot in the conventional searcher is determined by a voice activity, namely, a ratio of the voiced period to a whole time.
Moreover, such a useless searching operation in the searcher brings about deteriorating the quality of communication due to interference and noise caused to occur in the unvoiced period or slots.
Herein, consideration will be made about operation which is carried out in a mobile terminal during soft handoff procedure from a current base station to a new base station. In this event, the new base station can not detect a location of the mobile terminal before demodulation of a data signal arranged in the traffic channel, because of absence of any access channel in the reverse link. During the handoff procedure or operation, it is very important to accurately acquire a desired slot and to detect a data signal arranged in the traffic channel of the desired slot, so as to maintain the quality of communication and to avoid a call drop.
As mentioned above, when a searching operation of the searcher is executed in connection with slots which include no data. superfluous interference takes place not only during the soft handoff procedure but also during a normal communication, which results in degradation of the quality of communication.
It is an object of this invention to provide a method of controlling a searcher, which is capable of reducing any interference and noise to improve quality of communication.
It is another object of this invention to provide a method of the type described, which can reduce an amount of calculation necessary for searching a desired slot and which can improve power efficiency.
It is still another object of this invention to provide a receiver section which can effectively control the searcher and which can realize a high quality of communication.
It is yet another object of this invention to provide a CDMA system which can effectively search a desired slot A method to which this invention is applicable is for use in controlling a searcher in a CDMA system to receive a sequence of baseband reception data signals divisible into a plurality of frames each of which is divided into a plurality of slots and which has a frame rate determined by the number of the slots including the baseband reception data signals. According to an aspect of this invention, the method comprises the steps of calculating, in response to the baseband reception data signals arranged in a current one of the frames, burst candidate slot information representative of candidate slots in a next one of the frames following the current frame and controlling the searcher by the burst candidate slot information to make the searcher execute search operation within only the candidate slots in the next frame.
According to another aspect of this invention, the method comprises the steps of using a data burst randomizer to determine candidate slots which include the baseband reception data signals in a next one of the frames and operating the searcher during the candidate slots to make the searcher execute search operation within only the candidate slots in the next frame.
According to still another aspect of this invention, a receiver section is for use in a CDMA system to respond to a sequence of baseband reception data signals and to control a searcher included in a despread demodulator, the transmission data signal sequence being divisible into a plurality of frames each of which is divided into a plurality of slots and which has a frame rate determined by the number of the slots including the baseband reception data signals. The receiver section comprises a calculator, operable in response to the transmission data signal related to a predetermined one of the frames, for calculating candidate slots in a next frame following the predetermined frame and a control portion for controlling the searcher to make the searcher execute search operation within only the candidate slots in the next frame. dr
FIG. 1 shows a block diagram of a receiver section according to a first embodiment of this invention:
FIG. 2 shows a flow chart for use in describing operation of a next frame search rate judgment portion illustrated in FIG. 1;
FIG. 3 shows a block diagram of a demodulation control portion used in the receiver section illustrated in FIG. 1;
FIG. 4 shows a flow chart for use in describing production processing of a burst enable signal shown in FIG. 1;
FIG. 5 shows another flow chart for describing another production processing of the burst enable signal;
FIG. 6 shows a block diagram of a receiver section according to a second embodiment of this invention;
FIG. 7 shows a block diagram of a demodulation control portion illustrated in FIG. 6: and
FIG. 8 shows a flow chart for use in describing operation of a next frame set rate judgment portion illustrated in FIG. 6.